With the rapid development of mobile communications technologies, mobile communications network systems of multiple standards emerge, for example, a Global System for Mobile Communications (Global System for Mobile communication, GSM) network, a General Packet Radio Service (General Packet Radio Service, GPRS) system, a Wideband Code Division Multiple Access (Wideband Code Division Multiple Access, WCDMA) network, a CDMA-2000 network, a Time Division-Synchronous Code Division Multiple Access (Time Division-Synchronous Code Division Multiple Access, TD-SCDMA) network, and a Worldwide Interoperability for Microwave Access (Worldwide Interoperability for Microwave Access, WiMAX) network. In addition to voice communication services, generally, these mobile communications network systems also provide data communication services, so that users may use the data communication services provided by these mobile communications systems to upload and download various types of data.
When an environment in which a user equipment is located deteriorates, throughput and/or reliability of data transmission of the user equipment declines abruptly, and a multi-user cooperative communications (Multiple User Cooperative Communication, MUCC) technology may be used to support the data transmission of the user equipment. In MUCC, because the cooperative layer is located above a Packet Data Convergence Protocol (Packet Data Convergence Protocol, PDCP) layer, split data (including user plane data and possible control plane data (where the control plane data may be transmitted through a direct link between a benefited device and a base station, that is, not through a supporting device; and may also be transmitted through a link between the supporting device and the base station)) of the benefited device needs to pass through the PDCP layer of the supporting device before reaching the cooperative layer: Uplink data of the benefited device needs to pass through a short-distance communication layer between the supporting device and the benefited device, and after being processed by the benefited device at the cooperative layer, is forwarded to the base station by the support UE through a Uu interface, while downlink data of the benefited device needs to pass through the Uu interface (needing to pass through the PDCP layer) between the base station and the supporting device, and after being processed by the supporting device at the cooperative layer, is delivered to the benefited device through the short-distance communication layer between the supporting device and the benefited device.
To ensure security of data transmission between the benefited device, the supporting device, and the base station, the PDCP layer in an existing LTE mechanism may perform compression on an IP header of a data packet and perform encryption on data in the data packet (including encryption on user plane data, and encryption and integrity protection on control plane data). In MUCC, if a PDCP-layer key of the benefited device is used to perform encryption, that is, the base station uses the PDCP-layer key of the benefited device to encrypt data in a downlink direction, while the benefited device uses the PDCP-layer key of the benefited device to encrypt data in an uplink direction, the supporting device cannot successfully decrypt data of the benefited device at the PDCP layer, so that the supporting device cannot process data to be forwarded to the benefited device and forward data to be forwarded to a network device to the network side. In addition, security encryption protection needs to be performed on the data of the benefited device, so as to prevent the supporting device from obtaining information of the benefited device.